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February 17 2020

Städte für Menschen by Jan Gehl Alexander wants to read Städte für Menschen by Jan Gehl

January 18 2020

How To by Randall Munroe Alexander is on page 10 of 320 of <a href="/book/show/43852758-how-to">How To</a>.

August 18 2019

Meine Olympiade by Ilija Trojanow Alexander is on page 233 of 336 of <a href="/book/show/30648328-meine-olympiade">Meine Olympiade</a>.

August 07 2019

August 02 2019

Meine Olympiade by Ilija Trojanow Alexander is on page 145 of 336 of <a href="/book/show/30648328-meine-olympiade">Meine Olympiade</a>.

July 29 2019

March 07 2019

Troubleshooting nextcloud running on lighttpd

For historic reasons my personal nextcloud runs on lighttpd. (In fact I started in 2014 with ownCloud on Debian 8 (Jessie) and never changed the webserver.) This works, although it is not a first citizen supported platform, but after all nextcloud just needs some PHP/MySQL, so the webserver should not matter, should it?

After upgrading to nextcloud 15 and installing the social app/plugin, I got some warnings about missing rewrites/redirects. So it seems there are some new rewrites for service discovery in town, you can learn about those on the troubleshooting page of the administration manual.

You get config snippets for Apache and nginx (which are by far the most used webservers, I don’t blame nextcloud for not giving advise on every exotic webserver out there). For lighttpd you have to do it on your own and this is what I came up with. Load mod_redirect and mod_rewrite in your main lighttpd.conf and for the nextcloud config add this:

# for social app
        # TODO  After upgrade to lighttpd > 1.4.50 revisit
        #       https://redmine.lighttpd.net/projects/1/wiki/docs_modrewrite
        url.rewrite-once = (
                # Match when there are no query variables
                "^/.well-known/webfinger$" => "/public.php?service=webfinger",
                # Match query variables and append with &
                "^/.well-known/webfinger\?(.*)$" => "/public.php?service=webfinger&$1"
        )

        url.redirect = (
                "^/.well-known/caldav$" => "/remote.php/dav",
                "^/.well-known/carddav$" => "/remote.php/dav"
        )

If you installed in a subfolder (the subfolder is still called owncloud here, adapt if needed):

# for social app
        # TODO  After upgrade to lighttpd > 1.4.50 revisit
        #       https://redmine.lighttpd.net/projects/1/wiki/docs_modrewrite
        url.rewrite-once = (
                # Match when there are no query variables
                "^/owncloud/.well-known/webfinger$" => "/owncloud/public.php?service=webfinger",
                # Match query variables and append with &
                "^/owncloud/.well-known/webfinger\?(.*)$" => "/owncloud/public.php?service=webfinger&$1"
        )

        url.redirect = (
                "^/owncloud/.well-known/caldav$" => "/owncloud/remote.php/dav",
                "^/owncloud/.well-known/carddav$" => "/owncloud/remote.php/dav"
        )

For the future I consider changing the webserver, because lighttpd is not recommended by SabreDAV, which is used by nextcloud. But that’s not decided yet, for today this is it.

January 29 2019

December 19 2018

Wireshark USB capture setup with groups and udev

Wireshark does not only capture network traffic, but also different things like USB traffic. I needed that today and it needs some additional setup on Linux. There’s something in the Wireshark wiki on that topic, but I consider that not an elegant solution: USB capture setup.

The solution I use is basically one proposed on stackoverflow and uses a separate Linux system group and udev: usbmon (wireshark, tshark) for regular user.

On Debian you can do this:

addgroup usbmon
addgroup adahl usbmon

You have to log off and on again, check if you are in that group with the command id.

Now create a new file /etc/udev/rules.d/75-usbmon.rules and put this into it:

SUBSYSTEM=="usbmon", GROUP="usbmon", MODE="640"

After doing modprobe usbmon your devices /dev/usbmon* should belong to the new usbmon group and you can start capturing things with Wireshark.

November 23 2018

ISS Timelapse

Der italienische Astronaut Riccardo Rossi hat ein Timelapse-Video vom Start eines Progress-Frachters zur Internationalen Raumstation (ISS) gemacht und dieses Video ist wundervoll. Unbedingt im Vollbild ansehen!

October 24 2018

SSH Remote Capture mit Wireshark

Manchmal will man ja mal auf einem entfernten Gerät oder einem Embedded Board den Netzwerktraffic beobachten. Auf meinem PC würde ich dazu Wireshark nehmen, das läuft so natürlich nicht auf einem Gerät ohne grafische Oberfläche. Üblicherweise kommt dort tcpdump zum Einsatz, dann allerdings mit sehr viel weniger Komfort als man von Wireshark gewohnt ist.

Einen Tipp für eine komfortablere Variante gab es im RFC-Podcast Folge 12: RFCE012: IP Routing III + in eigener Sache.

Man kann nämlich den Traffic auf dem fraglichen Gerät aufzeichnen und live zu Wireshark auf dem PC rüber schubsen, bspw. mit SSH.

Eine übliche Variante, die schon seit langem funktioniert, ist folgende. Man startet auf dem PC in der Konsole dies hier:

ssh root@192.168.10.113 "/usr/sbin/tcpdump -i eth0 -U -w - 'not (host 192.168.10.62 and port 22)'" | wireshark -i - -k

Was bedeutet das? Es wird auf dem PC der SSH-Client aufgerufen und angewiesen sich als Nutzer ‘root’ mit dem Rechner auf der IP 192.168.10.113 zu verbinden. Dort soll er dann das Programm tcpdump mit gewissen Optionen starten. Ich rufe hier tcpdump mit vollem Pfad auf, weil es sein kann, dass es sonst nicht gefunden wird. Den Output davon bekomme ich im PC auf stdout und pipe den dann nach Wireshark, die entsprechenden Optionen bei dessen Aufruf bewirken, dass er die Daten auch verarbeiten kann und damit sofort loslegt.

Bisschen knifflig sind die Optionen von tcpdump, daher im einzelnen:

-i interface

Netzwerk-Schnittstelle, auf der tcpdump lauschen soll

-U
“Packet buffered” Output, d.h. tcpdump sammelt nicht, sondern schickt den Output pro Paket raus
-w file
tcpdump schreibt keinen lesbaren Output auf die Konsole, sondern ein maschinenlesbares Format in eine Datei, in diesem Fall nach stdout
filter expression
Hier will man den Traffic der SSH-Verbindung selbst natürlich ausfiltern, vorsichtig sein mit den Klammern, die müssen entweder escapet werden oder man schließt den Filterausdruck in Hochkommata ein.

Soweit so gut. In neueren Versionen von Wireshark gibt es noch die Variante, das direkt aus dem GUI heraus aufzurufen. Die entsprechenden Optionen sind leider nicht so super dokumentiert. Was für mich funktioniert, sieht man im folgenden Screenshot:

Screenshot Wireshark SSH Remote Capture Setup

Einige leere Optionsfelder sehen so aus wie Argumente für tcpdump, die scheint Wireshark zumindest in der hier gerade eingesetzten Version v2.6.3 nicht zu berücksichtigen, daher auch Remote Interface und Remote Capture Filter mit in der Zeile Remote Capture Command.

October 03 2018

October 01 2018

Alleingang Nanga Parbat by Reinhold Messner Alexander gave 3 stars to Alleingang Nanga Parbat by Reinhold Messner

July 09 2018

Getting CDash to work on Debian 9 (stretch) with lighttpd and MariaDB

After reading It’s Time To Do CMake Right and The Ultimate Guide to Modern CMake I stumbled about the slides of Effective CMake by Daniel Pfeifer. (I did not watch the related video C++Now 2017: Daniel Pfeifer “Effective CMake” though.)

What attracted my attention where the commands ctest_coverage() and ctest_memcheck() from the slide about CTest which comes with CMake and which I already use. In libcgi and some non free projects I create additional tests to be run with valgrind if that tool is found on the build host, but when using CTest/CDash I don’t need to do that and also get coverage tests on top, so I set up a local CDash server on my workstation, which was painful in multiple ways.

After extracting the CDash archive and configuring lighttpd to server its PHP files the install.php came back with the following error message:

Specified key was too long; max key length is 767 bytes

It was not easy to find the cause. The web says this is fixed in MariaDB 10.2.x while my Debian stable still has 10.1.x … and I found only some workarounds on that problem for other projects than CDash. I could “solve” that by changing the database collation from utf8mb4_unicode_ci to utf8_unicode_ci in phpMyAdmin on the still empty database cdash before running install.php.

The more challenging problem was to actually submit results to the CDash server when calling CTest. The webserver always responded with HTTP Status Code 417. That was only partly fault of CDash, which seems to call curl with some strange (?) headers for submission. That turned out to trigger some (from my side) unexpected behavior in lighttpd, for which several tickets exist, I found #1017 eventually, which led me to the lighttpd 1.4.21 release info giving the hint I needed. I added this somewhere in my lighttpd config files:

server.reject-expect-100-with-417 = "disable"

In the end I have a local CDash instance now and could already submit some helpful coverage and memcheck results. Best thing: This way I can remove the error prone additional tests from my CMakeLists.txt and still run the tests with valgrind, even more flexible than ever.

June 21 2018

stunning-palm-tree

So, what is this all about? Every now and then I want to try if I can solve some programming problem and make a tiny new project for that. For convenience I publish them on GitHub so I can clone them and play with them from multiple computers. Maybe someone else finds it and it helps someone, who knows.

I could name those tiny (not so) throw away projects like “testthis” or “evaluatethat” or come up with some more or less matching name, but GitHub has a nice feature when creating new repositories, GitHub makes a naming suggestion and I just pick that. See:

Screenshot GitHub new repository creation

Create a new repository on GitHub

Over time the number of those projects grew and to not lose track of them, here is a list:

May 30 2018

Bridges with meaningful error messages

Just wanted to create a network bridge interface on an embedded Linux system. First try was with the well known brctl and I got this:

$ brctl addbr br0
add bridge failed: Package not installed

Searching the web for this, led me to an old blog post which comes to this conclusion:

So that’s the most silly way to say: you forgot to compile in bridge support into your kernel.

ip from busybox is a little more helpful on that:

$ /sbin/ip link add name br0 type bridge
ip: RTNETLINK answers: Operation not supported

And yes, the “real” ip from iproute2 is also as helpful as that:

$ /usr/sbin/ip link add name br0 type bridge
RTNETLINK answers: Operation not supported

And yes, somehow they are all right, it is actually my fault:

# CONFIG_BRIDGE is not set

I’m going to change my kernel config now …

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May 13 2018

April 15 2018

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